Hyperacute motion detection by the lateral eyes of jumping spiders

Jumping spiders (Salticidae) are renowned for their high performing visual system. In addition to their prominent forward-facing telescope-like principal eyes, salticids possess two or three pairs of secondary eyes used for wide-angle motion detection. Salticids orient towards relevant sources of motion detected by the secondary eyes, enabling them to inspect the stimulus with their spatially acute principal eyes. The anteriormost pair of secondary eyes, the anterior lateral (AL) eyes, also faces forward and has higher spatial acuity than the other, laterally-facing, secondary eyes. We used small computer-generated targets to elicit orienting saccades from tethered jumping spiders in order to examine the perceptual limits of the AL eyes. We describe the contrast thresholds of male and female spiders, investigate the reaction time between stimulus appearance and initiation of orientation, as well as the minimum distance a stimulus must travel before eliciting a saccade. Our results show that female spiders react to lower contrast stimuli than males and demonstrate that the secondary eyes can detect stimulus displacements considerably smaller than the inter-receptor angle.